James Webb telescope

[mikewof]

In some ways, yes.  I've been involved with the Allen Telescope Array at Hat Creek, and an array of small dish antennas does have many advantages over a large antenna.  Advantages include cost (of course), wider field of view, better angular resolution, multi-beam capability (using phased array techniques), and others.  But the big antennas (and mirrors) still have an advantage when it comes to noise floor, which limits the ability to detect remote and weak sources.

I see the need ... the taxpaying public wants to see their data in the visible spectrum, and exoplanets are the trend. But how much mileage can we actually get from counting Earth-like planets? If we expect to make any progress in our ability to genuinely interact with the universe -- i.e. move around it -- we need to look far outside of the visible spectra. We need arrays to understand the real meat of the universe, like neutrino oscillations, weak interactions and universe expansion/contraction.

At some point, these exoplanets become like avoiding a relationship with the wife in order to spend all day at the strip club where we can look but not touch.

 

[valis]

I see the need ... the taxpaying public wants to see their data in the visible spectrum, and exoplanets are the trend. But how much mileage can we actually get from counting Earth-like planets? If we expect to make any progress in our ability to genuinely interact with the universe -- i.e. move around it -- we need to look far outside of the visible spectra. We need arrays to understand the real meat of the universe, like neutrino oscillations, weak interactions and universe expansion/contraction.

At some point, these exoplanets become like avoiding a relationship with the wife in order to spend all day at the strip club where we can look but not touch.

I think we should do both -- wives and strip clubs.  I mean, multiple types of sensing capabilities.  And isn't the Webb mostly IR, not visible spectrum?

 

[Bus Driver]

Webb Update



Yesterday finished a great day with the completion of sunshield, DRSA-V, and STSA deployments (all covered in yesterday’s post). Today’s plan is deceptively simple in description but of monumental importance. That is the deployment of the Secondary Mirror Support Structure (SMSS). These are the long arms that are currently folded up against the large primary mirror. They will come down and lock the telescope’s secondary mirror into position far out in front of the primary mirror. Light enters the telescope from the front and reflects off the large primary mirror, goes up to the small secondary mirror where it then bounces back into a hole in the center of the primary mirror and then into the science instruments. Obviously, a properly positioned secondary mirror is critical to the telescope working.



Stay tuned… and enjoy the ride

 

[Bus Driver]

And isn't the Webb mostly IR, not visible spectrum?

It is.

 

[Voiled]

In some ways, yes.  I've been involved with the Allen Telescope Array at Hat Creek, and an array of small dish antennas does have many advantages over a large antenna.  Advantages include cost (of course), wider field of view, better angular resolution, multi-beam capability (using phased array techniques), and others.

I've been told Telescope Arrays need to be extremely accurately positioned relative to each other to produce data that can be combined to something meaningful. And that such accuracy currently is not possible with space-based telescopes. Can you/anyone confirm?

 

[valis]

I've been told Telescope Arrays need to be extremely accurately positioned relative to each other to produce data that can be combined to something meaningful. And that such accuracy currently is not possible with space-based telescopes. Can you/anyone confirm?

I can only guess, but I think you're right.  Radio telescopes operate in the MHz-GHz range (the Allen Telescope Array is designed for 500 MHz to 10 GHz).  At 1 GHz the wavelength is 300 cm, so a 1 GHz array needs 3D precision accuracy of perhaps 30 cm (and 3 cm for 100 GHz).  The frequency of visible light is around 500 THz (wavelength = 600 nanometers).  So we're talking about perhaps 60 nm accuracy in positioning of the detectors.  This is why multi-element optical telescope reflectors are so incredibly finicky.  And the optical array "processing" is done with optics, which need to maintain coherence from one end to the other.  With radio we can to some extent compensate for detector position drift using adaptive delays in the combining / processing stages, but with multi-element optical reflectors we need to actively control the element position before we can use the combined image.  From the Webb Telescope page: 

Aligning the primary mirror segments as though they are a single large mirror means each mirror is aligned to 1/10,000th the thickness of a human hair

Getting out my micrometer, a hair is about 75 um (micro meters), so 1/10000 of that is 7.5 nm.  So I was only off by a factor of ten -- it's hard.

I was about to talk about the bandwidth requirements for distributed array datalinks, but I sense that my audience is already falling asleep.  Anyway, it's a hard enough problem on land, and much harder than that in space.

 

[Charlie Foxtrot]

Hey Driver! Maybe I missed that day in school, but it looks to me like the JWST's telescope is fixed to point ~90 degrees from the short axis of the spacecraft.  That means that imaging will be relatively limited to a circular wedge maybe 45 degrees? above the long axis with the ability of the craft to rotate about the short axis. Not an issue, as the photons have been waiting around for billions of years.    It would mean that it would take about a half year to completely sweep the sky.      

Astrophysics was never my strong subject in grade school - am I correct?  

Thanks!

 

[mikewof]

I think we should do both -- wives and strip clubs.  I mean, multiple types of sensing capabilities.  And isn't the Webb mostly IR, not visible spectrum?

Visible definitely sells the magazines, but the budgets are limited and we aren't leveraging our ability to make dirt cheap sensors in huge arrays ... we could manage synthetic apertures of thousands of kilometers.

My limited knowledge of the Webb is that it goes from about the wavelength of green light up into the microwave spectra. With synthetic apertures we could image from microwave way, way up into meter-length and even kilometer-length spectra ... incredibly low-energy light. It could transform our understanding of the universe's clockwork.

 

[mikewof]

I've been told Telescope Arrays need to be extremely accurately positioned relative to each other to produce data that can be combined to something meaningful. And that such accuracy currently is not possible with space-based telescopes. Can you/anyone confirm?

As far as I know, meter-level positioning is possible and this was already pitched to NASA some time ago but without any ensuing budget, it just didn't light any fires of taxpayer interest. The proposal was an interferometric array somewhere of the coast of Saturn. The later proposal was one past the Moon, which would do the double duty of using the Moon as a type of Neutrino weak-interaction absorber.

 

[valis]

Visible definitely sells the magazines, but the budgets are limited and we aren't leveraging our ability to make dirt cheap sensors in huge arrays ... we could manage synthetic apertures of thousands of kilometers.

My limited knowledge of the Webb is that it goes from about the wavelength of green light up into the microwave spectra. With synthetic apertures we could image from microwave way, way up into meter-length and even kilometer-length spectra ... incredibly low-energy light. It could transform our understanding of the universe's clockwork.

Space-based low-frequency synthetic aperture might be doable, if my DRE (*) analysis above is correct.  But "kilometer-length spectra" being light?  Or are those two separate things?

(*) DRE = technical acronym for "Direct Rectal Extraction", i.e. "pulled it out of my ass" 

 

[IStream]

Webb was designed for a specific mission: to look as far back in time as possible. That means seeing dim objects through a lot of dust, which means putting the biggest IR photon bucket out that that you can and (to Valis's point) pushing the sensor noise floor down as far as possible. 

When you do spectroscopy, you spread your finite photon collection over more noisy sensor elements. Same thing with arrays. Yes, in exchange you get spectral info and/or spatial resolution, but that's not as important for Webb's mission.

 

[Voiled]

The podcast with an astronomer i was listing to was also talking about nanometer scale accuracy needed.

 

[valis]

I've always thought that space (and the Moon) would be a great place for low-frequency radio transmitter work.  We could build vacuum tubes using trash cans and chicken wire, and mile-long travelling wave tubes.

 

[mikewof]

Space-based low-frequency synthetic aperture might be doable, if my DRE (*) analysis above is correct.  But "kilometer-length spectra" being light?  Or are those two separate things?

(*) DRE = technical acronym for "Direct Rectal Extraction", i.e. "pulled it out of my ass" 

"Light" is all spectra, from the crazy high energies of gamma rays and x-rays (Angstrom-length scales) up to visible light (hundreds of nanometers) up to microwaves and then the low energies of meter lengths up to ELF (extremely low frequency) light with wavelengths of hundreds of kilometers and higher.

The apertures required to sense these incredibly low energies aren't really viable on Earth, but it should work in the stability and limitless positioning precision of distant Earth orbits. China is already doing this, they have already launched a three-monopole system: https://arxiv.org/abs/1909.08951, while we do essentially nothing in this area that isn't directly related to communicating with submarines. It seems that in the USA, if we can't pull a pretty picture of a planet somewhere, or an exoplanet, that there is no hope for actually funding such a thing.

We will eventually need to understand the process of neutrino oscillations if we hope to couple weakly to the varying mass that they produce, and then somehow use that mass for space propulsion sp we can use the limitless mass in the universe's neutrino field for propulsion, rather than drag our propulsion mass up from the Earth as we currently do, like the bunch of ill-educated orangutans that we are ... akin to throwing gold bars off of a sled to move across a frozen lake.

 

[Charlie Foxtrot]

Nevermind... Good stuff from Nasa:   https://ntrs.nasa.gov/api/citations/20090026783/downloads/20090026783.pdf



They're able to pitch the spacecraft 50 degrees while remaining in the shadow to the sun shield.  Wowza!     

 

[d'ranger]

Not sure its been mentioned but the Canadians and Europeans are partners in this and the reason it was launched by the French Ariane 5 was when being developed it was the largest available so everything was designed to fit inside. The launch location was an additional benefit that escapes me at the moment.

I would appreciate if those who chose to respond to you-know-who don't quote him - it's not necessary and keeps down the clutter. TIA.

 

[valis]

The launch location was an additional benefit that escapes me at the moment.

Benefit being the equatorial launch location, possibly helping achieve the solar orbit?  Or something else?

 

[Rain Man]

Not sure its been mentioned but the Canadians and Europeans are partners in this and the reason it was launched by the French Ariane 5 was when being developed it was the largest available so everything was designed to fit inside. The launch location was an additional benefit that escapes me at the moment.

I would appreciate if those who chose to respond to you-know-who don't quote him - it's not necessary and keeps down the clutter. TIA.

Asians launches from Guyana near the equator.  That takes advantage of the earth's rotation (466 m/s at the equator) to reduce launch energy requirements and makes the transition to orbit easier.

 

[Rain Man]

Ariane not "Asians". Stupid autocorrect and short edit window...

 

[DA-WOODY]

I would appreciate if those who chose to respond to you-know-who don't quote him

@ least ^^ that's ^^ passive

looking around these forums and the number of "Pressure Drop" like Hate comments

it quite likely would be a Bad Idea to exchange communications with more advanced Life forms out in space

the closest anyone on earth these days are to getting along - is banding together against others

Those who allow us to find them could come to see earth as the Wuhan of Space

(if that wasn't why they put us here in the 1st place)

and keep us from spreading by Re-Setting Earth back to Day 1

if we don't do it 1st